please help with 89 f150 sputtering

I have a 1989 ford f150 v8. tuned it up and ran great. changed the MAP sensor and it ran good . she starts great and idles fine until you get to about 40-45 miles an hour then she acts like shes not getting fuel i checked the pressure and its fine also changed the fuel filter on the rail it was black as can be but pulled it off again and know it looks good. any ideas. NO check enngine light or anything like i said runs great until you get up there and then she wants to loose power and stall out. :runaround:

Could be a bad coolant tempature sensor,reads the tempature wrong and will do that with

changed the coolant temp sensor and it still does the same thing. How many fuel injectors does this truck have and is there a way to test them.

It has 8 of them and you do not want to replace them without further testing,$100.00 a piece.

I hope you've found the problem by now but could it be a clogged cat?